As used herein the term "rotor" means a device designed to rotate continuously, such as the rotating portion of an electric, hydraulic or pneumatic motor, the shaft of a motor or other mechanical mechanism, an idler shaft, a drive shaft, etc.
An unbalanced rotor, i.e., a rotor having a mass distribution that is unbalanced relative to the axis-of-rotation of the rotor, can cause structural vibrations. In addition to undesirable noises, the structural vibrations resulting from an unbalanced rotor cause fatigue in the rotating machine and any structure to which the rotor is coupled. Normally, the rotors of rotating machines are balanced during their manufacture. Unfortunately, under some circumstances, rotors can become unbalanced. When a rotor becomes excessively unbalanced, it must be rebalanced. The classical way of balancing rotors consists of adding and/or removing weights at particular rotor axial locations so as to make the rotor symmetric, in terms of mass, about the rotor's axis-of-rotation. Such balancing techniques can be costly and time-consuming. Further, such balancing techniques can result in a rotor which is only balanced for a particular set of operating conditions. More specifically, the balance of some rotors varies with rotational frequency. Classical balancing does not take rotational frequency variations into consideration. Rather, balancing is carried out at a specific rotational frequency, which means that the rotor may be unbalanced at other frequencies.
Take, for example, the balancing of the shafts of jet engines in an aircraft. While jet engine shafts are balanced when manufactured and during periodic maintenance of the aircraft, new imbalances frequently develop due to such things as minor damage to the turbine blades of the jet engines. Imbalances in jet engine shafts produce repetitive vibrations in jet engine housings, engine mounts, and the wings and fuselage of the associated aircraft. Repetitive vibrations cause fatigue in components of an aircraft, which reduces the life of the aircraft and could result in an in-flight disaster. Further, the repetitive vibrations resulting from shaft imbalances affect jet engine auxiliary and suspension equipment such that these components require regular maintenance. In addition to affecting safety and maintenance cost, repetitive vibrations produced by unbalanced jet engine shafts create noise in the passenger cabin of the aircraft. Current procedures for rebalancing jet engine shafts are very costly. Also, the balancing procedures only balance jet engines for certain operating conditions, e.g., a certain combination of temperature and rotational frequency.
In the past, passive means have been utilized to reduce the effect of unbalanced rotors. For example, vibration absorbing suspension systems have been used to mount rotating machines. Such suspension systems can considerably reduce the effect of an unbalanced rotor on surrounding structure. Unfortunately, vibration absorbing suspension systems can introduce additional problems, such as allowing relative motion betwen a rotating machine and the structure to which it is fixed when subjected to static and slow dynamic loads.
Neither manually rebalancing the rotational elements of rotating machines or using passive means to reduce the effect of imbalances is completely satisfactory. In between rebalancing, rotating machines operate with any imbalances developed since the most recent rebalancing. Further, rebalancing may be only effective for a given set of operating conditions. Passive means can counter the effect of imbalances in rotors, but there is a limit to the effectiveness of passively reducing the effect of imbalanced rotors. The present invention provides an apparatus for continuously actively balancing a rotor by automatically controlling the mass distribution of the rotor relative to the axis-of-rotation about which the rotor rotates. The active system iteractively improves the balance of the rotor whether or not changes occur in operating conditions.